This invention relates to guidance devices for self-powered vehicles and more particularly to an improvement for a magnetic wire-following guidance system for a vehicle.
In designing an electronic vehicle guidance system which follow an energized wire buried in the floor, the conventional approach is to use two sensor coils mounted on the vehicle and which normally straddle the buried wire. An alternating current in the buried wire induces voltages in the sensing coils which varies with the spacing of the vehicle center from the buried wire. The difference between the sensor coil signals gives a linear indication of the vehicle centering error which is used to control an electromechanical steering mechanism to keep the vehicle centered over the wire. Such a system is disclosed in the applicant's U.S. Pat. No. 4,040,500 mentioned above.
When the amount of centering error is greater than half of the spacing between the sensors, the output from the sensors starts to decrease with increasing centering error. Since lead networks needed for servo stabilization make the steering correction highly sensitive to the rate of change of error, this incorrect slope causes the steering mechanism to correct in the wrong direction. For example, at a certain distance from the buried wire, the slope of the rate of change of the sensor signals indicates a decreasing error signal even though the vehicle is actually moving away from the wire. Normally, the sensor signals would give a decreasing error signal when the vehicle was moving towards the wire. It should be noted that the error signal in question is the difference between the two sensor outputs. When the vehicle is beyond a certain predetermined distance from the wire the guidance system may actually cause a steering correction in the wrong direction because of the negative slope of the rate of change of the error signals.
In the applicant's guidance system as described in the aforementioned patent, the vehicle can be manually steered over the buried wire and then control can be relinquished by the opertor to the automatic guidance system. In such a system the width of the linear sensor characteristic is particularly important since the vehicle will naturally overshoot the wire somewhat during the transition between manual and automatic control. If the overshoot is more than half of the sensor spacing, the reverse slope area of the sensor characteristic will be entered and the vehicle will steer itself off of the wire. If the sensor characteristic is widened by simply increasing the spacing between the sensors, the guidance accuracy suffers because the signal ouput of the sensors is greatly reduced when the vehicle is centered over the wire. The linearity of the sensor characteristics and the signal-to-noise ratio also suffer when the spacing between the sensors is increased.